1. Field of the Invention
This invention relates to pulse-width modulation (PWM) circuits for use in power amplifier circuits such as self-excited class-D amplifiers. In addition, this invention also relates to power amplifier circuits using pulse-width modulation circuits.
2. Description of the Related Art
FIG. 10 shows an example of the configuration of a power amplifier circuit using a pulse-width modulation (PWM) circuit. Herein, an input signal such as an audio signal given from a signal source 31 is supplied to a pulse-width modulation circuit 33 in which it is subjected to pulse-width modulation. That is, the pulse-width modulation circuit 33 produces a PWM signal for driving a switching circuit 34. The switched output of the switching circuit 34 is supplied to a speaker 36 via an LC filter 35.
The pulse-width modulation circuit 33 contains an integrator 39 consisting of an operational amplifier 38 and a capacitor 38, and a comparator 43 consisting of an operation amplifier 40 and resistors 41 and 42. Herein, the integrator 39 and the comparator 43 having hysteresis characteristics are connected together in a cascade-connection manner, wherein the comparator 43 provides a PWM signal that is fed back to the integrator 39 via a resistor 44. The switching circuit 34 comprises an n-type field-effect transistor (FET) 45 and a p-type field-effect transistor (FET) 46. The LC filter 35 comprises an inductor (or inductance) 47 and a capacitor (or capacitance) 48.
The pulse-width modulation circuit 33 outputs a pulse signal having a duty ratio 50% unless no input signal is applied thereto. When the input signal is applied to the pulse-width modulation circuit 33, the integrator 39 provides the integrated output thereof that is shown in FIG. 11. The integrated output is supplied to the comparator 43 having hysteresis characteristics, which in turn produces a PWM signal resulting from the pulse-width modulation that is effected on the pulse signal in response to the input signal. In response to the PWM signal, the switching circuit 34 alternately switches on or off the FETs 45 and 46, so that the PWM signal is amplified and is then supplied to the speaker 36 via the LC filter 35. Thus, the speaker 36 is driven by the xe2x80x98amplifiedxe2x80x99 PWM signal.
If the speaker 36 is directly driven by the PWM signal, the carrier component of the PWM signal must be sent into the speaker 36 to deteriorate an efficiency in transducer, or it may destroy the speaker 36. For this reason, the LC filter 35 is interposed to eliminate the carrier component of the PWM signal.
The aforementioned power amplifier circuit using the pulse-width modulation circuit acts as a self-excited class-D amplifier, which may generally provide a xe2x80x98flatxe2x80x99 frequency characteristic shown in FIG. 12. Herein, the slope of the frequency characteristic curve may depend upon the characteristic and the order of the integrator. This circuit can effectively eliminate the carrier component of the PWM signal to provide a xe2x80x98sharpxe2x80x99 frequency characteristic close to that of the multi-order low-pass filter, for example.
Power amplifier circuits for use in audio devices are required to provide a low distortion ratio and low output impedance in order to eliminate unwanted influences to the sound quality. In the case of the power amplifier circuit using a linear amplifier, negative feedback is arranged with respect to the output side of the amplifier; therefore, it is possible to extremely reduce the distortion and output impedance in the output side of the amplifier. In the case of the power amplifier circuit using a pulse-width modulation circuit, it may be preferable to arrange negative feedback in the output side of the pulse-width modulation circuit. However, this power amplifier circuit operates based on the xe2x80x98analogxe2x80x99 input signal, while the PWM signal output from the pulse-width modulation circuit is a digital signal, which cannot be directly subjected to negative feedback. Therefore, this power amplifier circuit requires an LC filter as the low-pass filter for converting the PWM signal to the analog signal, wherein the output of the LC filter would be subjected to negative feedback. However, when the output of the LC filter is subjected to negative feedback, the LC filter may cause the distortion and the increase of the output impedance.
The LC filter is constituted in the second order comprising a single inductor and a single capacitor. Alternatively, it can be constituted in higher orders. The LC filter of the second order may produce phase revolution of 180xc2x0 in the high frequencies range. In addition, the pulse-width modulation circuit inevitably provides the low-pass filter characteristic due to the integrator therein. Therefore, the signal transmitted through the LC filter provides phase revolution of 180xc2x0 or more. Hence, there is a problem that the analog signal output from the LC filter cannot be subjected to negative feedback to the input signal.
In addition, the power amplifier circuit of FIG. 10 has the following problems:
That is, the gain G is produced between the input terminal P of the comparator 43 and the output terminal Q of the switching circuit 34, wherein the signal amplitude xe2x80x98VP-VMxe2x80x99 (where xe2x80x98VPxe2x80x99 denotes the maximal value of the integrated output input to the input terminal P; and xe2x80x98VMxe2x80x99 denotes the minimal value) emerges at the input terminal P, while the signal amplitude xe2x80x98VPX-VMXxe2x80x99 (where xe2x80x98VPXxe2x80x99 denotes the positive source voltage applied to the switching circuit 34; and xe2x80x98VMXxe2x80x99 denotes the negative source voltage) emerges at the output terminal Q. Herein, the gain G can be calculated in accordance with the following equation (1).                     G        =                                            V              PX                        -                          V              MX                                                          V              P                        -                          V              M                                                          (        1        )            
In the above equation (1), there is a possibility that each of the source voltages VPX and VMX may be varied. When each of them is varied, the gain G must be varied. As a result, the system stability of the power amplifier circuit should be altered. Thus, it must be required to further raise the system stability above the prescribed requirement.
It is an object of the invention to provide a power amplifier circuit using a pulse-width modulation circuit that can reduce phase revolution to secure negative feedback in a stable manner.
It is another object of the invention to provide a power amplifier circuit that guarantees the stable operation by suppressing variations of the gain thereof.
A pulse-width modulation circuit of this invention comprises a comparator whose noninverting input receives an input signal to output a pulse-width modulation (PWM) signal, which is fed back to the noninverting input to provide hysteresis characteristics, and an integrator that integrates the pulse-width modulation signal and negatively feeds it back to the inverting input of the comparator.
A power amplifier circuit is constituted using the aforementioned pulse-width modulation circuit. In addition, a switching circuit is arranged to amplify the pulse-width modulation signal based on the positive source voltage (VPX) and negative source voltage (VMX). The amplified pulse-width modulation signal is supplied to a speaker via an LC filter, the output of which is negatively fed back to the pulse-width modulation circuit.
In the above, it is possible to supply the noninverting input of the comparator with either a fraction of the positive source voltage or a fraction of the negative source. Further, a triangular wave generation circuit is arranged inside of the pulse-width modulation circuit to provide a triangular waveform whose maximal and minimal values substantially match (specific fractions) of the positive and negative source voltages.
Since the pulse-width modulation signal whose phase is advanced is transmitted through the LC filter, it is possible to reduce phase revolution in the output of the power amplifier circuit. Thus, it is possible to effect negative feedback on the pulse-width modulation signal in a stable manner.